1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and a method of fabricating an LCD device, and more particularly, to an LCD device having a temperature measurement device and a method of fabricating an LCD device having a temperature measurement device.
2. Discussion of the Related Art
As demand for information increases, significant effort has been made to develop various types of flat display devices, such as liquid crystal display (LCD) devices, plasma display panel (PDP) devices, electroluminescent display (ELD) devices, and vacuum fluorescent display (VFD) devices. Among the various types of flat display devices, LCD devices have been commonly used to replace cathode ray tube (CRT) devices due to the LCD device's thin profile, light weight, and low power consumption. In addition to mobile-type LCD devices, such as displays for notebook computers, LCD devices have been developed for use as computer monitors and televisions.
Despite various technical developments within the LCD technology field, image quality enhancement of the LCD devices has been lacking in comparison to other features and advantages of the LCD devices. In order to use LCD devices as general display devices, the LCD devices must be able to produce high quality images, such as high resolution and luminance, with large-sized screens, while still maintaining their light weight, thin profile, and low power consumption.
In general, an LCD device includes an LCD panel for displaying images, and a driving part for supplying driving signals to the LCD panel. The LCD panel includes first and second glass substrates bonded to each other having a predetermined interval therebetween, wherein a liquid crystal material is injected between the first and second glass substrates. The first glass substrate (i.e, TFT array substrate) includes a plurality of gate and data lines, a plurality of pixel electrodes, and a plurality of thin film transistors (TFTs). The plurality of gate lines are formed on the first glass substrate at fixed intervals, and the plurality of data lines are formed perpendicular to the plurality of gate lines at fixed intervals. Then, the plurality of pixel electrodes are arranged in a matrix-type configuration and are each formed within pixel regions defined by crossings of the plurality of gate and data lines. The plurality of TFTs are switched ON/OFF according to signals transmitted along the gate lines in order to convey signals transmitted along the data lines to the pixel electrodes. The second glass substrate (i.e., color filter substrate) includes a black matrix layer that excludes light from regions except for the pixel regions of the first substrate, R/G/B color filter layers for producing colored light, and a common electrode to produce images in combination with the pixel electrode.
The LCD device is driven according to optical anisotropy and polarizability of liquid crystal molecules in the liquid crystal material, wherein the liquid crystal molecules are aligned by their long and thin shapes. Accordingly, an induced electric field is supplied to the liquid crystal material for controlling alignment directions of the liquid crystal molecules. For example, if the alignment direction of the liquid crystal molecules is controlled by the induced electric field, light is polarized and changed due to the optical anisotropy of the liquid crystal material, thereby displaying images. Presently, Active-Matrix type LCD devices are being developed that include arrangements of TFTs and pixel electrodes connected thereto in a matrix-configuration to provide high resolution and improved image quality.
FIG. 1 is a perspective view of an LCD device according to the related art. In FIG. 1, an LCD device includes lower and upper substrates 1 and 2, and a liquid crystal layer 3 formed by injecting liquid crystal material between the lower and upper substrates 1 and 2. The lower substrate 1 includes a plurality of gate lines 4 disposed along one direction at fixed intervals, a plurality of data lines 5 perpendicular to the gate lines 4 to define pixel regions P, pixel electrodes 6 in the pixel regions P defined by crossing the gate and data lines 4 and 5, and TFTs T at respective crossing portions of the gate and data lines 4 and 5. The upper substrate 2 includes a black matrix layer 7 for preventing light leakage on portions except the pixel regions P, RIG/B color filter layers 8 for producing colored light, and a common electrode 9 for producing images in combination with the pixel electrode 6.
Although not shown, the TFT T includes a gate electrode protruding from the gate line 4, a gate insulating layer formed along an entire surface of the substrate, an active layer on the gate insulating layer above the gate electrode, a source electrode protruding from the data line 5, and a drain electrode being opposite to the source electrode. The pixel electrode 6 is formed of transparent conductive metal having high light transmittance, such as indium-tin-oxide (ITO). In the LCD device, liquid crystal molecules of the liquid crystal layer 3 positioned on the pixel electrode 6 are aligned by signals supplied via the TFT T, whereby images are displayed by controlling light transmittance due to the alignment of the liquid crystal molecules of the liquid crystal layer 3.
FIG. 2 is a graph showing a relationship between temperature and dielectric constant of liquid crystal material according to the related art. In FIG. 2, as temperature increases, dielectric constant ∈(⊥) of the liquid crystal molecules along the short axis (minor axis) increases, and dielectric constant ∈(//) of the liquid crystal molecules along the long axis (major axis) decreases, thereby increasing dielectric anisotropy. In addition, as the temperature increases, modulus of elasticity of the liquid crystal material decreases. Accordingly, the decrease in the modulus of elasticity and the dielectric anisotropy of the liquid crystal material results in difficulties in aligning the liquid crystal molecules along predetermined directions during application of voltages when the temperature of the LCD panel is high.
In FIG. 2, the dielectric constant of the liquid crystal material is at a normal temperature of 20° C., or more. Although not shown, if the temperature of the LCD panel is reduced to below 0° C., some of the liquid crystal material within the LCD panel is changed into a crystalline state, thereby deteriorating fluidity of the liquid crystal material. Accordingly, when applying the voltages, it is difficult to obtain an alignment state of the liquid crystal molecules along a desired direction. For example, when the liquid crystal material of the LCD panel is maintained at relatively high or low temperatures, control of the alignment of the liquid crystal molecules becomes difficult before and after application of the voltage. Thus, measuring and compensating for operational temperatures of the LCD panel must be performed.
Generally, the LCD device displays images by using the liquid crystal material having an intermediate state somewhere between the fluid state and the solid state. For example, the LCD device displays the images due to maintaining the liquid crystal material in the fluid state by using the optical anisotropy of the liquid crystal material. Accordingly, the light transmittance of the liquid crystal material is changed according to the operation temperature of the LCD device, wherein the liquid crystal material has the highest light transmittance at a temperature of about 20° C.
FIG. 3 is a graph showing a relationship between temperature and drain current when turning OFF a TFT according to the related art, and FIG. 4 is a graph showing a relationship between temperature and drain current when turning ON a TFT according to the related art. In FIG. 3, during the turn OFF state of the TFT, wherein the TFT supplies a constant gate voltage (−10V) to a gate, a drain current value increases non-linearly and the temperature of the TFT increases slightly. In FIG. 4, during the turn ON state of the TFT, wherein the TFT supplies a constant gate voltage (20V) to a gate, a drain current value linearly increases as the temperature increases. When the drain current flow exceeds a predetermined value, a pixel electrode connected to the TFT is turned ON. Accordingly, as the temperature of the TFT increases, the drain current increases, thereby turning ON the pixel electrode even though the applied gate voltage decreases at increased temperatures.
TFT-LCD devices for Car Navigation System (CNS) or for military equipment require wide operational temperature ranges for proper operation. Accordingly, since the current for turning ON/OFF the TFT increases as the temperature increases, information may, or may not be displayed by the TFT-LCD devices. One solution includes determining a capacitance of a storage capacitor of the pixel due to the operational temperature characteristics of the turn OFF current. Accordingly, since the LCD device has different driving characteristics according to changes in operational temperatures of the LCD device, a temperature sensor for sensing the temperature of the LCD panel is provided at an exterior portion of the LCD panel. However, providing the temperature sensor an the exterior of the LCD panel makes it difficult to sense the precise temperature of the LCD panel. In addition, since both the LCD panel and the temperature sensor are relatively expensive components, the temperature sensor is commonly produced inexpensively in order to reduce manufacturing costs.